The present invention relates generally to the field of heterostructure lasers and, more particularly, to a laser having matched quaternary active layers which give rise to a low threshold current and high thermal stability.
Optical communications systems and fiber sensors require light to be transmitted through long lengths of optical fiber. Common silica fibers exhibit low loss and low dispersion at wavelengths of 1.3-1.55 microns (0.95-1.1 electron volts). Thus far, it has not been possible in the GaAs/AlGaAs material system to produce lasers with bandgaps within this range. However, it is known that quaternary materials such as InGaAsP can be grown lattice-matched to InP within a broad bandgap range by independent variation of components controlling bandgap and lattice constant. Using these materials, one can produce devices having bandgaps which fall within the desired range for optical fibers.
Lasers based on InGaAsP/InP exhibit longer lifetimes and greater resistance to facet damage than their GaAs/AlGaAs counterparts, making them prime candidates for high power and long lifetime applications, such as optical communication. However, InGaAsP/InP lasers have heretofore been plagued with a temperature-sensitive threshold current which can lead to device failure in applications where heat sink temperatures approach 50 degrees Celsius. This temperature dependence is expressed mathematically as low values of T.sub.o in the exponential expression for threshold current (J.sub.th =J.sub.tho e.sup.T/To). It has been the subject of considerable investigation and results in large part from carrier leakage over the barrier at the heterostructure of the laser. Because the energy of the carriers increases exponentially with temperature, one would expect carrier leakage to increase dramatically with temperature. Electron leakage dominates in these devices because the effective mass of electrons is an order of magnitude smaller than that of holes.
A quaternary heterostructure laser which is useful in the 1.3 micron range and significantly reduces carrier leakage is disclosed by Yano et al., Appl. Phys. Lett. 41 (5), 390 (1982) and Yano et al. IEEE Journal of Quantum Electronics, QE-19 No. 8, 1319 (1983). The Yano device, called a "double carrier confinement" (DCC) laser, employs two active layers of InGaAsP one p-type and the other either n-type or undoped, separated by a thin p-type InP separation layer. Electrons which leak from the first active layer diffuse across the separation layer and have a second chance to radiatively recombine in the second active region. Yano et al. disclose DCC lasers with T.sub.o values of 180 degrees Kelvin and thresholds of 4.6 kA/cm.sup.2. This T.sub.o value is 3 times larger than that of a typical 1.3 micron laser, but the threshold current is undesirably large. Devices with such large threshold currents are inefficient in operation and are prone to thermal runaway.